Some metals can be recovered from feedstocks such as ores, concentrates, mattes or alloys by processes which include an electrowinning step. Examples of such processes include the recovery of cobalt and nickel from feedstocks containing sulfides or oxides of these metals.
Electrowinning involves subjecting an electrolyte containing dissolved metal salts to electrolysis. Electrowinning is conducted in one or more electrolytic cells, each cell having a plurality of anode and cathode plates in alternating arrangement. During the electrowinning step, elemental metal is plated out at the cathode and, where the metal salt is a sulfate, for example, oxygen is evolved at the anode. The oxygen gas is evolved from the anode in the form of bubbles which rise to the surface of the electrolyte and burst. The bursting bubbles release the electrolyte into the atmosphere above the tank in the form of a fine mist or spray. This acidic mist is corrosive and hazardous to health of workers in the electrowinning tankhouse.
Hydrogen ions are also produced at the anode. In conventional cells, some of these hydrogen ions migrate to the cathode where they combine with electrons to produce hydrogen gas. The consumption of electrons by hydrogen ions can significantly reduce current efficiency in the electrolytic cell.
It is known to enclose the cathodes and/or anodes of the electrolytic cells in “bags”, or in compartments separated by membranes or porous diaphragms, in order to improve current efficiency and to prevent generation of electrolyte mist in the space above the electrolytic cells. Examples of bagging technology are shown in U.S. Pat. No. 2,321,367 (Diggin), U.S. Pat. No. 4,075,069 (Shinohara et al.) and U.S. Pat. No. 6,120,658 (Dunn et al.).
The use of mist-reducing technologies such as bagging and compartmentalized cells is gaining increasing acceptance in various types of electrowinning circuits. In fact, anode bagging is considered to be highly desirable for nickel recovery and useful for cobalt recovery. The benefits of such technologies are often sufficient to convince designers and operators of electrowinning facilities to incorporate such technologies in new installations. However, the application of mist-reducing technology to older facilities can be challenging and expensive, and the benefits are usually insufficient to justify the capital commitment needed to modify existing infrastructures to accommodate the technology.
Therefore, there is a need for improved metal recovery processes which will encourage the use of mist-reducing technology in existing facilities and which will improve the efficiency and working conditions of both new and existing facilities.